1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system and, in particular, to an apparatus capable of switching a pixel density of an electrostatic latent image.
2. Related Background Art
In recent years, image forming apparatus utilizing an electrophotographic system to obtain an image of high quality have been widely used. However, as a way to realize an image of much higher quality, the image forming apparatus provided with improved resolution, that is, high pixel density of 600 and 1,200 or further 2,400 dpi is desired. In order to deal with diversified prints, an apparatus capable of switching the pixel density of the electrostatic latent image by only one image forming apparatus so that the optimum image can be obtained according to a print purpose has been put to practice.
By the way, as a method of setting up an image forming condition according to resolution, in addition to a method of switching a spot size (a size in which the output of an exposing beam is equal to 1/e2 of the maximum value), there have been various proposals made such as setting up an effective spot size of the exposing beam according to resolution (Japanese Patent Application Laid-Open No. 60-31167), switching a duty ratio (Japanese Patent Application Laid-Open No. 3-87773), changing exposure strength of the exposing beam according to resolution (Japanese Patent Application Laid-Open No. 4-322270), switching an amount of light by using a high sensitive drum (Japanese Patent Application Laid-Open No. 10-274868) and the like.
However, in the conventional image forming apparatus using such methods, when a switching of the image density is performed in one image forming apparatus, there has been some cases where, when either one of a line width of a letter image and the density of a solid black image or a dot image is adjusted, the image quality of the other becomes varied, thereby leading to a difference in an image repeatability.
That is, usually there are many cases where the image forming condition is matched so that the density of the line width of the letter image and the solid black image becomes the same. However, when the condition is matched in such a way, the dot image becomes thinner in density for high resolution. For example, when the pixel density is changed from 600 dpi to 1,200 dpi, if the image forming condition is matched so that the line width becomes the same, there have been the cases where the density of the dot image becomes thinner for 2xc3x972 dots of 1,200 dpi (equivalent to 1 dot of 600 dpi) than for 1 dot of 600 dpi. On the contrary, when the image forming condition is matched so that the dot image becomes the same, the line width having high resolution becomes thicker.
An object of the present invention is to provide an image forming apparatus capable of forming the same images in both letters, solid black images and dot images even when a pixel density is changed.
Another object of the present invention is to provide an image forming apparatus comprising an image bearing body and exposing means for exposing the charged image bearing body based on an image signal and forming an electrostatic latent image on the image bearing body, wherein it is possible to switch a pixel density of the electrostatic latent image. Control means controls a responsiveness to the image signal of the exposing means when the pixel density is switched.
Still another object of the present invention will be apparent from the following description.